Such devices are used, in general, for compacting granular or powdery vegetable materials, such as cocoa or coffee, but also for teas and infusions to compact them prior to fill them into bags. For in packaging such bulk material, the problem arises that the bag should contain a predetermined weight, on the one hand, and that the bag should be well filled, on the other hand. This requires a predetermined bulk weight of the material, i.e. a predetermined weight/volume ratio (=specific density). This is especially a problem if different sorts of material, such as coffee, having undergone different degrees of treatment, such as different degrees of roasting or having a different fineness after milling, shall be packed, and overfilling or underfilling of the package should be avoided.
In a known device of this kind (see EP-A-0 485 772), a single shaft having, first, a feeding screw and then compacting tools comprising stirring and compacting paddles are provided, an adjustable outlet opening being postponed for final compaction. The size of the outlet opening is merely manually adjustable in order to cause the retaining effect requested and the correlated change of pressure.
It has now been found that, although such an arrangement meets the purpose, it involves also some disadvantages:
1. The throughput is determined by the number of revolutions, the pitch and the diameter of the feeding screw. With a given screw geometry, however, solely one number of revolutions will make sense. On the other hand, in case that another number of revolutions, e.g. a higher one, is required for the compaction unit, the feeding screw would produce unnecessary frictional work due to its retaining effect, thus heating the material in an undesirable manner. PA1 2. The outlet cross-section, which remains fixed once it has been adjusted, does not guarantee a constant pressure, that is to say in case the throughput is not constant, i.e. with an increase of the throughput, the pressure within the apparatus will raise too, and vice-versa.